1. Field of the Invention
The invention relates to an internal combustion engine equipped with a control mechanism for controlling the timing of opening/closing the intake and exhaust valves and to a control method thereof.
2. Description of the Related Art
It is known that in the internal combustion engines, intake air and exhaust air pulsate in their respective pipes in association with the opening and closing actions of valves and the actions of pistons. Such intake pressure pulsation and exhaust pressure pulsation typically occur due to “reflection” of pressure changes caused by the intake valves and the exhaust valves, and such reflection can occur at a surge tank, a catalytic converter, etc., which are provided in the intake and exhaust passageways, more specifically at their openings or portions having an increased-sectional area. If the cycle of such intake and exhaust pulsation and the opening/closing timing of the intake and exhaust valves are synchronized, the inflow of fresh air from the intake pipe into the combustion chambers will be accelerated so as to improve the intake charge efficiency, and the efficiency of discharging burned gas from the combustion chambers into the exhaust pipe will be improved.
In association with this, there are known technologies in which intake pulsation and intake inertia effect are utilized to adjust the intake charge efficiency so as to increase the engine output. A known internal combustion engine that adopts such a technology includes a valve characteristic switching mechanism for controlling the cam operation angle by switching between two cams provided on an intake camshaft, a valve phase varying mechanism capable of changing the opening/closing timing of the intake valves to a retarded side and an advanced side by controlling the rotation phase of the intake camshaft, and intake pipe length changing mechanism for changing the characteristic frequency of pressure waves of intake pulsation and thereby adjusting the synchronization characteristic by changing the length of the intake pipe. In this internal combustion engine, intake pulsation and the intake inertia effect corresponding to the engine rotation speed can be achieved by changing the operation angle of the intake cams stepwise, and changing the phase of the intake cams (to the retarded or advanced side), and changing the period of intake pulsation stepwise. Thus, this internal combustion engine is advantageous in increasing the output. As a patent literature that describes the foregoing internal combustion engine technology, the following Patent Literature 1 is cited.
[Patent Literature 1]
Japanese Patent Application Laid-Open Publication No. 2000-328971
With regard to the exhaust side, a technology is known in which the exhaust pressure is detected and the opening/closing timing of the valves is accordingly set so as to improve the scavenging efficiency that depends upon exhaust pulsation, and therefore improve the intake charge efficiency. As a patent literature that describes such an internal combustion engine technology, the following Patent Literature 2 is cited.
[Patent Literature 2]
Japanese Patent Application Laid-Open Publication No. 11-22499
However, a problem with the above-described internal combustion engines is that it is difficult to always realize optimal valve timing. That is, even if the intake cam phase is set based on experimental values or design values so as to achieve an optimal valve timing in a certain operation region, that timing can not always be optimal in fact since the manner or characteristic of the above-stated intake pulsation changes due to aging changes or individual differences of the engine, etc. Furthermore, if cams are employed to open and close the valves, a correlation exists between the opening timing and the closing timing of the valves, so that the regions where the intake pulsation is sufficiently utilized during various states of engine operation are limited. That is, there are many regions where the dynamic effect of intake cannot be fully utilized due to insufficient response to variations of intake pulsation depending on the state of operation.